Recently, there have advanced increase in screen size, high resolution, and high frame rate of display devices such as television receivers (hereinafter, referred to also as televisions). Televisions capable of displaying an image so that a viewer can view the image three-dimensionally (that is, carrying out 3D display) have been in widespread use.
FIG. 19 is a table showing an example of scan periods of one scan line in each of display devices that are different in resolution and the number of scan lines from one another, the each of display devices being driven at different frame frequencies.
As shown in FIG. 19, a scan period Ton of one scan line is shortened as the resolution increases, that is, as the number of scan lines increases. The scan period Ton of one scan line is also shortened as the frame frequency increases, that is, as a frame rate increases.
FIG. 20 is a table showing an example of a relationship among (i) a resistance of one scan line, (ii) a capacitance of the one scan line and (ii) a time constant with respect to each screen size of the display device whose resolution is FullHD (see FIG. 19), the display device being driven.
As shown in FIG. 20, the resistance and the capacitance of the one scan line, and the time constant are increased as the screen size increases.
As is clear from FIGS. 19 and 20, in order to attain increase in screen size, high resolution, high frame rate, and 3D display of the display devices, the scan period of one scan line is shortened, and the time constant of the one scan line is increased. This causes a reduction in charging rate of a pixel electrode, whereby the pixel electrode is inadequately charged.
The following description will discuss, with reference to FIGS. 21 and 22, an example case where a pixel electrode results in being inadequately charged because a scan period of one scan line can be kept only short.
FIG. 21 is a timing chart illustrating ideal short waveforms of a scan signal and a video signal, which timing chart is obtained in a case where a conventional driving method, in which a scan period of one scan line can be kept only short, is employed. FIG. 22 is a view illustrating a change in charging voltage of each terminal of a TFT (thin film transistor) during a scan period Ton. Note that a scan signal is supplied to a gate terminal of the TFT via a corresponding scan line, and a video signal is supplied to a source terminal of the TFT via a corresponding video signal line.
As illustrated in FIG. 21, a scan signal Gn becomes Vgh during a scan period Ton in a frame. This causes the TFT to be turned ON during the scan period Ton. During the scan period Ton during which the TFT is in an ON state, a pixel electrode connected to a drain terminal of the TFT is charged to an electric potential (positive electric potential in FIG. 21) of a video signal to be supplied via the source terminal.
Since the scan period Ton is short, a gate waveform, indicative of an ultimate voltage (gate voltage) of the gate terminal, which ultimate voltage is charged due to a scan signal to be supplied to the gate terminal, does not reach Vgh but merely reaches Vgh′ (note here that Vgh′<Vgh) during the scan period Ton (see FIG. 22). This causes a drain voltage Vd, which is an ultimate voltage of the drain terminal (that is, an ultimate voltage of the pixel electrode connected to the drain terminal), to fail to reach an ultimate voltage (source voltage) Vs of the source terminal to be charged due to a video signal. Accordingly, the pixel electrode is not adequately charged. Such charging shortage of the pixel electrode attributes to lack of performance of the TFT which electrically connects the source terminal and the drain terminal to each other, the lack of performance being caused by no reach of the ultimate voltage of the gate terminal up to Vgh.
Patent Literature 1 describes a technique of preventing a drain terminal from being inadequately charged (see FIG. 24) by preliminarily charging a pixel electrode before charging (primary charging) the pixel electrode (see FIG. 23). FIG. 23 is a timing chart of a scan signal and a video signal, which timing chart is obtained in a case where the technique described in Patent Literature 1 is employed. FIG. 24 is a view illustrating a change in charging voltage of the drain terminal in a case where the technique described in Patent Literature 1 is employed.
Patent Literature 2 describes a technique of (i) setting not only a voltage which causes a TFT to be switched ON and a voltage which causes the TFT to be switched OFF but also an intermediate electric potential between the voltages (see FIG. 26) and (ii) consuming less electric power for charging a pixel electrode. FIG. 26 is a timing chart of a scan signal and a video signal, which timing chart is obtained in a case where the technique described in Patent Literature 2 is employed.
There has been devised a technique of starting to supply a scan signal to an (n+1)th scan line while scanning in which the scan signal is supplied to the (n+1)th scan line is being superimposed on scanning in which a scan signal is supplied to an nth scan line (see FIG. 27). This makes it possible to (i) keep a scan period Ton′ longer than a normal scan period Ton and (ii) cause an ultimate voltage of a gate terminal to get closer to Vgh (see FIG. 28). FIG. 27 is a timing chart of a scan signal and a video signal, which timing chart is obtained in a case where the technique is employed, in which technique the (n+1)th scan line is scanned so that a scan period of the (n+1)th scan line is superimposed on a scan period of the nth scan line. FIG. 28 is a view illustrating a change in voltage of the gate terminal in the case where the technique is employed, in which technique the (n+1)th scan line is scanned so that the scan period of the n+1th scan line is superimposed on the scan period of the nth scan line.